Many clinical conditions, deficiencies, and disease states can be remedied or alleviated by supplying to the patient a factor or factors produced by living cells or removing from the patient deleterious factors which are metabolized by living cells. In many cases, these factors can restore or compensate for the impairment or loss of organ or tissue function. Examples of disease or deficiency states whose etiologies include loss of secretory organ or tissue function include (a) diabetes, wherein the production of insulin by pancreatic islets of Langerhans is impaired or lost; (b) hypoparathyroidism, wherein the loss of production of parathyroid hormone causes serum calcium levels to drop, resulting in severe muscular tetany; (c) Parkinsonism, wherein dopamine production is diminished; and (d) anemia, which is characterized by the loss of production of red blood cells secondary to a deficiency in erythropoietin. The impairment or loss of organ or tissue function may result in the loss of additional metabolic functions. For example, in fulminant hepatic failure, liver tissue is rendered incapable of removing toxins, excreting the products of cell metabolism, and secreting essential products, such as albumin and Factor VIII. Bontempo, F. A., et al, (1987) Blood, 69, pp. 1721-1724.
In other cases, these factors are biological response modifiers, such as lymphokines or cytokines, which enhance the patient's immune system or act as anti-inflammatory agents. These can be particularly useful in individuals with a chronic parasitic or infectious disease, and may also be useful for the treatment of certain cancers.
It may also be desirable to supply trophic factors to a patient, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4/5), ciliary neurotrophic factor (CNTF), glial cell line-derived neurotrophic factor (GDNF), cholinergic differentiation factor/Leukemia inhibitory factor (CDF/LIF), epidermal growth factor (EGF), insulin-like growth factor (IGF), basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF) and the like.
In many disease or deficiency states, the affected organ or tissue is one which normally functions in a manner responsive to fluctuations in the levels of specific metabolites, thereby maintaining homeostasis. For example, the neurons of the hippocampus produce high levels of NGF which is directly supportive of the basal forebrain cholinergic neurons which innervate the hippocampus. A decrease in the level of NGF produced by these neurons may result in the loss of cholinergic input to this vital structure, resulting in age-associated memory impairment found in Alzheimer's disease.
In the nervous system, chronic, low-level delivery of trophic factors is sufficient to maintain the health of growth-factor dependent cell populations. In chronic disorders such as Alzheimer's disease and Huntington's disease, long-term delivery of one or more neurotrophic factors such as NGF, BDNF, NT-3, NT-4/5, CNTF, GDNF and CDF/LIF may be required to maintain neuronal viability. These growth factors cannot be delivered through systemic administration as they are unable to traverse the blood-brain barrier. Therefore, these neurotrophic factors must be delivered directly into the central nervous system (CNS).
Many investigators have attempted to reconstitute, augment, or replace organ or tissue function by transplanting whole organs, organ tissue, or cells which provide secreted products or affect metabolic functions. Moreover, transplantation can provide dramatic benefits but is limited in its application by the relatively small number of organs suitable and available for grafting. In general, the patient must be immunosuppressed in order to avert immunological rejection of the transplant, which generally results in loss of transplant function and eventual necrosis of the transplanted tissue or cells. In many cases, however, it is desireable for the transplant to remain functional for a long period of time, even for the remainder of the patient's lifetime. It is both undesirable and expensive to maintain a patient in an immunosuppressed state for a substantial period of time.
Another approach used in transplantation procedures is the implantation of cells or tissues within a semi-permeable physical barrier which will allow diffusion of nutrients, waste materials, and secreted products, but minimize the deleterious effects of the cellular and molecular effectors of immunological rejection. A variety of devices or capsules which protect tissues or cells producing a selected product from the immune system have been explored. These include extravascular diffusion chambers, intravascular diffusion chambers, intravascular ultrafiltration chambers, and implantation of microencapsulated cells (Scharp, World J. Surg., 8, pp. 221-9 (1984)). These devices were envisioned as providing a significant advance in the field of transplantation, as they would alleviate the need to maintain the patient in an immunosuppressed state, and would thereby allow many more patients to receive restorative or otherwise beneficial transplants by allowing the use of donor cells or tissue which could not have been used with the conventional transplantation techniques.
The use of encapsulated cells hinders elements of the immune system from entering the capsule, thereby protecting the encapsulated cells from immune destruction. This technology increases the diversity of cell types that can be employed in therapy. The semipermeable nature of the capsule membrane also permits the molecule of interest to easily diffuse from the capsule into the surrounding host tissue. This technique prevents the inherent risk of tumor formation and allows the use of unmatched human or even animal tissue, without immunosuppression of the recipient. Moreover, the implant may be retrieved if necessary or desired. Such retrievability may be essential in many clinical situations.
The outer surface morphology may affect a variety of parameters including the strength of the capsule, the retrievability of the capsule, as well as the ability of the capsule to support viable cells for extended periods of time.
It is desirable to provide capsules that permit viability of the encapsulated cells for extended periods of time and that are more easily retrievable without breakage.
Numerous encapsulation devices are known, having various outer surface morphologies. Capsules have been categorized as Type 1 (T1), Type 2 (T2) or Type 4 (T4) depending on their outer surface morphology. Such membranes are described, e.g., in Lacy et al., "Maintenance Of Normoglycemia In Diabetic Mice By Subcutaneous Xenografts Of Encapsulated Islets", Science, 254, pp. 1782-84 (1991) and Dionne et al., PCT/US92/03327. The novel membranes of this invention have been designated T1/2, and are characterized by a hybrid outer surface morphology wherein the total area occupied by macropores, as well as the macropore diameter fall within a selected range.
The use of dividing cells and cell lines to provide the needed biological function offers a number of significant advantages over fully differentiated tissue and/or organs. Cells may be grown to large numbers in vitro and can be banked and screened for pathogens. Additionally, cells and cell-lines are more amenable to genetic engineering than primary organs, or tissues. The ability to introduce heterologous recombinant DNA allows many new possibilities for the alteration of the function or phenotype of cells to be transplanted. This in turn provides for a greater diversity of therapeutic uses for transplanted cells.
Retroviral vectors have generally been employed to genetically alter the cells used in such procedures (Gage et al., U.S. Pat. No. 5,082,670). However, it is known that retroviral expression vectors do not provide high-level long-term in vivo expression of heterologous proteins. A variety of factors contribute to the observed down-regulation of transgene expression under the control of retroviral promoters. These factors include quiescence of the genetically altered cells, methylation of CpG doublets within the promoters, and removal of selection pressure. Most expression vectors driven by mammalian promoters are also not best suited for traditional transplantation paradigms because of their inherent low-level promoter activity (See M. Schinstine and F. Gage, Molecular and Cellular Approaches for the Treatment of Neurological Disease, S. G. Waxman, ed., Raven Press pp. 311-323 (1993)).
In addition to the problem of down regulation of retroviral promoters in the CNS, there are other disadvantages in using retroviruses for gene therapy. For example, there is a serious concern about the possibility for recombination events occurring within a transplanted mammalian host previously exposed to or currently infected with virus containing genetic elements which may result in the conversion of replication-defective virus to live virus. In addition, working with infectious virus particles poses safety risks for the laboratory workers and medical practitioners producing and administering the reagent. Finally, these concerns have led to a heightened perception of risk among researchers and medical practitioners as well as regulatory authorities.
Although genetically engineered cells have been transplanted in vitro both in encapsulated and unencapsulated form, long-term, stable expression of the heterologous DNA has not been satisfactorily achieved. For example, a recently published study (Hoffman et al., Experimental Neurology, 122, pp. 100-106 (August, 1993)) refers to the use of encapsulated, allogeneic cells genetically engineered to secrete mouse-NGF for the delivery of NGF to the central nervous system (CNS) of rats. The NGF gene expression in the described system was under the control of a retroviral promoter. As described above, retroviral vectors do not give rise to long-term, stable expression of transgenes in vivo. Accordingly, the method reported in that study will not be suitable for long-term therapeutic applications.
Accordingly, a method of delivering appropriate quantities of needed substances, such as growth factors, enzymes and hormones, from genetically altered cells, for an extended period of time is still unavailable and would be very advantageous to those in need of long-term treatment. Moreover, methods for maintaining the long term, stable in vivo expression of transgenes in transplanted cells are also unavailable and are needed (for example, see Schinstine and Gage (1993), supra, at p. 321).
Therefore, the need remains for devices and delivery methods which incorporate genetically altered cells that facilitate long-term, stable transgene expression in vivo.